Silica-free UV-absorbing abrasion resistant coatings

ABSTRACT

A silica-free, abrasion resistant and ultraviolet radiation resistant coating is disclosed comprising an organic UV absorbing compound such as hydroxybenzophenone in an inorganic oxide matrix formed by the hydrolysis and condensation of an organo alkoxysilane and a silicon other metal alkoxide.

FIELD OF THE INVENTION

The present invention relates generally to protective coatings for plastics such as polycarbonate, and more particularly to coatings which protect the underlying substrate from damage caused by exposure to ultraviolet radiation.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,275,118 to Baney et al. discloses a coating composition comprising an acidic dispersion of colloidal titania, colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium which produces a hard, abrasion-resistant coating when cured on a plastic surface such as polycarbonate.

U.S. Pat. No. 4,299,746 to Frye discloses an improved silicone resin abrasion resistant coating composition prepared by hydrolyzing an alkyltrialkoxysilane in an aqueous colloidal silica dispersion and adding thereto a small amount of an ultraviolet light absorbing compound such as 2,4-dihydroxybenzophenone to the hydrolysis product.

U.S. Pat. Nos. 4,390,373 and 4,442,168 to White et al. disclose a cured, transparent, abrasion resistant coating composition comprising an effective abrasion resisting amount of a colloidal dispersion containing colloidal antimony oxide and colloidal silica in a weight ratio of less than or equal to about 30:70 in a water-alcohol solution of the partial condensate of R (Si(OH)₃) wherein R is an organic radical.

U.S. Pat. No. 4,395,461 to Ching discloses a method for improving the weatherability of a polycarbonate substrate by initially treating the surface of the polycarbonate substrate with a solution of a hydrolyzed silylated organic UV screen followed by the application of a curable silicone hardcoat composition onto the treated substrate.

U.S. Pat. No. 4,405,679 to Fujioka et al. discloses a coated shaped article of a polycarbonate type resin of improved abrasion resistance comprising a shaped polycarbonate substrate, an undercoat applied and cured on the substrate, and an overcoat applied and cured on the undercoat comprising a hydrolyzate of an epoxy-containing silicon compound, at least one member of the group of hydrolyzates of organic silicon compounds, colloidal silica and organic titania compounds, and a curing catalyst.

U.S. Pat. No. 4,477,499 to Doin etal. discloses ultraviolet radiation resistant silicone resin coatings having improved thermoformability and shortened required aging achieved by the addition of a Lewis acid compound to the coating composition.

U.S. Pat. No. 4,477,519 to Frye discloses a dual component coating system useful on transparent plastic substrates which includes a primer coat of polymethylmethacrylate in a solution of ethylene glycol monoethyl ether and diacetone alcohol containing a small amount of an ultraviolet light absorbing compound, and a second coat of silicone resin comprising a dispersion of colloidal silica and a different ultraviolet light absorbent compound.

U.S. Pat. Nos. 4,500,669 and 4,571,365 to Ashlock et al. disclose transparent, abrasion resistant coating compositions comprising a colloidal dispersion of a water insoluble dispersant in a water-alcohol solution of the partial condensate of silanol wherein the dispersant comprises metals, alloys, salts, oxides and hydroxides thereof.

U.S. Pat. No. 4,552,936 to Moore discloses a protective coating composition containing a polycaprolactone polyol and an aminoplast derivative applied to a thermoplastic substrate to produce a thermoformable article having abrasion resistance, solvent resistance and weatherability.

European Patent Application No. 851102939 published May 2, 1986 entitled "Carbon-Containing Monolithic Classes Prepared by a Sol-Gel Process" by Baney et al. of Dow Corning Corporation discloses an intermediate which comprises a dispersion of a colloidal metal oxide in a water-alcohol solution of the partial condensate of a silanol having the formula RSi(OH)₃, wherein the metal oxide is ZrO₂, SnO₂, ZrSiO₄, B₂ O₃ or La₂ O₃.

Optical quality abrasion resistant coated plastic materials generally require a coating that protects the substrate from the damaging effects of ultraviolet (UV) radiation. Protection from ultraviolet radiation is especially important for polycarbonate, since hydrolytic degradation is apparently accelerated by UV exposure. Conventional UV stabilizers do not impart sufficient protective capacity to abrasion resistant coatings, as sufficient amounts of most typical organic UV absorbers cannot be added to water/alcohol based sol-gel type abrasion resistant coating compositions due to limited solubility and further without adversely affecting hardness and adhesion of the coating. Moreover, typical UV absorbers may gradually become deactivated after prolonged exposure, and also may gradually be leached from the composition.

SUMMARY OF THE INVENTION

The present invention provides protection for underlying plastic substrates such as polycarbonate from damaging ultraviolet radiation by means of transparent coatings containing ultraviolet absorbing organic compounds in partially hydrolyzed sols of alkoxides of silicon and/or other metals which hydrolyze and polymerize by condensation to form a silica-free film which provides abrasion resistance to the plastic substrate surface. Such ultraviolet radiation (UV) absorbing compounds as hydroxybenzotriazole and hydroxybenzophenone are compatible with alcoholic/aqueous sol-gel compositions and processes, and provide UV protection without compromising abrasion resistance of the inorganic oxide coating. The coatings of the present invention may further comprise an organic film-forming polymer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Coatings that protect plastic substrates such as polycarbonate from damage caused by ultraviolet radiation are formed from sols containing network-forming metal alkoxides such as alkoxysilanes and other metal alkoxides. The alkoxysilane may be an organoalkoxysilane, such as an alkylalkoxysilane or other organofunctional alkoxysilane. The alkoxide may contain alkyl or aryl groups and may be in dimer or higher condensed form so long as hydrolyzable alkoxide groups remain. An ultraviolet radiation absorbing organic compound is added to the partially hydrolyzed sol. Hydrolysis and condensation polymerization occur in situ. A preferred catalyst is tetramethyl ammonium hydroxide. Coating compositions are preferably prepared by hydrolyzing an alkyl trialkoxysilane, an epoxy-functional trialkoxysilane or dialkyl dialkoxysilane with an ultraviolet light absorbing compound. Alternatively, the alkoxide may be partially or fully hydrolyzed, and condensed to some extent prior to combination with the organic UV absorber, preferably 2,4-hydroxybenzophenone. An organic film-forming polymer may be added, preferably polyvinylpyrrolidone.

The resulting composition may be used as a coating composition for either a primer or a protective overcoat on a substrate or coated substrate. In an alternative embodiment, the composition may be added to other coating compositions, such as abrasion resistant coating compositions, to increase their resistance to ultraviolet radiation. In either case, the coating compositions may be applied by any conventional technique, such as spraying, dipping or flow coating. The coating is preferably cured for about 45 minutes to 2 hours at a temperature of about 120° to 130° C. The composition dries and cures to form a uniform durable coating with good adhesion to plastic substrates such as polycarbonate. Preferred coating thickness is about 3 to 6 microns, preferably about 3.4 to 4.5 microns. The coating protects the substrate from ultraviolet radiation by providing strong and broad absorption in the 240 to 380 nanometer range.

In a preferred embodiment of the present invention, an alkoxide is partially hydrolyzed before adding an ultraviolet absorbing organic compound containing hydrophilic extensions with terminal hydroxyl groups capable of reacting with silanol groups. Preferably, the alkoxide is an alkoxysilane of the general formula R_(x) Si(OR')_(4-x) wherein R is an organic radical, R' is selected from the group consisting of low molecular weight alkyl radicals, and x is less than 4 and may be zero. The organic radical of R is preferably alkyl, vinyl, methoxyethyl, phenyl, γ-glycidoxypropyl, or γ-methacryloxypropyl. The alkoxide hydrolyzes according to the general reaction

    R.sub.x Si(OR').sub.4-x +yH.sub.2 O→R.sub.x Si(OR').sub.4-x-y (OH).sub.y +yR'OH.

Condensation of the hydrolyzed alkoxide proceeds according to the general reactions ##STR1## Further hydrolysis and condensation follow.

The pH and/or degree of condensation of the alkoxides may be adjusted, if necessary, to prevent haze or precipitation upon mixing with the organic ultraviolet absorber, preferably 2,4-hydroxybenzophenone. The sol preferably comprises about 0.5 to 5.0 percent by solid weight of the UV absorber. Alkoxides of titanium and/or zirconium may also be included in compositions of the present invention. The ultraviolet radiation protection provided by the 2,4-hydroxybenzophenone containing coating of the present invention may be determined by measuring the UV absorbence spectrum of the coating applied on a quartz substrate.

The silicon alkoxide portion of the composition preferably comprises 1 to 20 percent by weight of a trialkoxysilane RSi(OR')₃, where R is preferably a short to medium chain length organic group such as vinyl, phenyl, 3-glycidoxypropyl, or 3-methacryloxypropyl, preferably 3-glycidoxypropyl. The presence of a small amount of trialkoxysilane other than alkyl trialkoxysilane such as CH₃ Si(OR)₃ is important to the formation of hydrolytic polycondensation products that provide both the excellent abrasion resistance and hardness of coatings that contain colloidal silica and the excellent flexibility, thermal formability and resistance to cracking during exposure to thermal and mechanical stress that are provided by coatings that use mixtures of alkyl trialkoxysilanes and dialkyl dialkoxysilanes. The latter tend to sacrifice some abrasion resistance and hardness, even with the use of colloidal silica filler.

The use of a mixture of short alkyl- and longer organofunctional trialkoxysilanes results in the following advantages for preparing protective coatings for transparent plastics that are both abrasion resistant and flexible. The polysiloxane network itself provides the abrasion resistant character of the coating. It is not necessary to provide substance to the polysiloxane network by using a "filler" material such as colloidal silica that allows buildup of sufficient film thickness of the siloxane coating to achieve abrasion resistant character without cracking of the film.

Preferred coating compositions in accordance with the present invention comprise 28 to 48 percent alkyltrialkoxysilane, preferably methyl trimethoxysilane; 1 to 20 percent organofunctional alkoxysilane, preferably γ-glycidoxy trimethoxysilane; 20 to 40 percent water; 1 to 3 percent acid, preferably glacial acetic acid; optionally, up to 1 percent organic polymer such as polyethylene oxide, preferably polyvinylpyrrolidone; up to 1 percent ultraviolet light absorber, preferably Uvinul 400; up to 0.1 percent curing catalysts such as organic or inorganic acetates, amines, carboxylates, hydroxides or similar basic salts, preferably up to 0.1 percent of sodium acetate and tetramethyl ammonium hydroxide.

In order to coat polycarbonate substrates with abrasion resistant sol-gel compositions of the present invention, preferably an acrylic primer solution is prepared as follows. A solution is prepared from 5 grams of methylmethacrylate/ethylmethacrylate copolymer of 60/40 ratio in warm solvent mixture of 65 grams of propylene glycol methyl ether (or Dowanol PM of Dow Chemicals) and 20 grams of diacetone alcohol. A solution of 0.50 gram Tinuvin 900 and 0.50 gram Tinuvin 1130 UV absorbers in 5 grams of toluene is added to the above resin solution, as taught in copending application Ser. No. 07/275,494 filed Nov. 21, 1988, by Linet al. entitled "UV Resistant Primer", the disclosure of which is incorporated herein by reference. Primer solutions can be applied by any conventional method such as dip, drain, flow or spray, and are preferably cured for 10 to 20 minutes at about 120° C.

ASTM F735-81 is used to determine the Bayer abrasion resistance of the coating. A 4"×4" coated polycarbonate is subjected to an oscillating silica sand box for 300 cycles of abrasion in 2 minutes. The change in the percentage of haze which is measured by a hazemeter (Hazegard system of Pacific Scientific) is reported. Typically, uncoated polycarbonate has a 55-60% haze increase in this test. Tabor abrasion testing is done in accordance with ASTM D1044 and percent haze after abrasion measured in accordance with ASTM D1003.

The UV resistance of the coating itself as well as its ability to protect an underlying substrate from damage by UV is evaluated by submitting the coated polycarbonates to an accelerated Q-UV chamber equipped with fluorescent tubes (B313 nanometers) manufactured by Q-panel Corporation according to the testing method described in ASTM G53-77. The chamber is programmed for an 8 hour light cycle at 70° C., followed by a 4 hour, 100% condensing cycle at 50° C. The appearance of the coating is inspected, and the discoloration reported in terms of the yellowness index (per ASTM E308-85, 1931 CIE, Illuminant C) using a UV/VIS spectrophotometer. Uncoated polycarbonate typically reaches a value of 10 soon after 100 hours of exposure.

The present invention will be further understood from the descriptions of specific examples which follow.

EXAMPLE I

A solution consisting of 80 grams of methyltrimethoxysilane (Union Carbide A-163), 2 grams of dimethyldiethoxysilane (A-164), 4 grams of glacial acetic acid and 3.0 grams of Uvinul 400 (BASF) is stirred at room temperature until the Uvinul 400 dissolved, and added with stirring to a room-temperature solution of 25 grams of methanol and 55 grams of deionized water. After stirring for 16 hours, the solution is diluted with 20 grams of 2-propanol, 20 grams of 1-butanol, 0.1 gram of BYK-300 flow control agent, and treated with 2 grams of BioRad AG 1-x8 anion exchange resin (hydroxide form) for 3 hours. This raises the sol pH from 3.25 to 4.2. Tetramethylammoniumhydroxide, 0.30 gram of 20 percent solution, is added as a cure catalyst, raising the solution pH to 5.1. Stirring at room temperature is continued to develop adhesion to a thermoset acrylic primer containing an additional ultraviolet light absorbing agent.

An acrylic primer solution is prepared by dissolving 60 grams of a 60/40 copolymer of poly(methylmethacrylate-ethylmethacrylate) (Polyscience) in 1082 grams of warm (75° C.) Dowanol PM. The copolymer solution is cooled to room temperature and 360 grams of diacetone alcohol, 86 grams of toluene, 15 drops of BYK-300 (BYK Chemie) and 13.5 grams of Tinuvin 900 are added with stirring until dissolution of the Tinuvin is complete. The resulting solution can be applied to polycarbonate or other plastic substrates by dip coating; for flow or spray application further dilution with Dowanol PM, diacetone alcohol and toluene is necessary to achieve proper thickness (0.5 to 2.0, preferably 1.0 micron).

Primed polycarbonate samples are dip-coated, dried for five minutes in air at ambient temperature and cured for 90 minutes at 130° C. The following table compares the test results of coated and uncoated samples.

                  TABLE I                                                          ______________________________________                                         Test     Duration Measurement   Coated                                                                               Uncoated                                 ______________________________________                                         Bayer    300      % Haze         10   55-60                                    Abrasion cycles                                                                Taber    300      % Haze        7-8   >55                                      Abrasion cycles                                                                QUV-FS-40                                                                               Hours    craze/debond  900   250 (10)                                 QUV-B313 Hours    craze/debond  700   100 (10)                                                   (yellowness index)                                                                           (>5)                                           ______________________________________                                    

EXAMPLE II

A solution consisting of 240 grams of methyltrimethoxysilane (Union Carbide A-163), 24 grams of 3-glycidoxytrimethoxysilane (A-187), 12 grams of glacial acetic acid, and 9 grams of Uvinul 400 (BASF) is added to a vigorously stirred solution of 75 grams of methanol and 1.5 grams of polyvinylpyrrolidone, PVP-K90 (mw 300,000), in 165 grams of deionized water at room temperature. Stirring is continued at room temperature for 1 day, after which the solution is treated with a mixture of 75 grams of 2-propanol, 37.5 grams of diacetone alcohol, and 0.6 gram of BYK-300 (BYK Chemie) flow control agent followed by 0.3 gram of Na(OAc).3H₂ O. After another five days of stirring, 1.5 grams of 20 percent aqueous tetramethylammonium hydroxide is added. After another two days, the sol is diluted with 15 parts ethyl acetate and 1 gram of BYK-300.

A thermoset acrylic primer containing an additional UV light absorbing agent is prepared by dissolving 64 grams of Elvacite 2041 PMMA (DuPont) in 1152 grams of warm (80° C.) Dowanol PM. Upon cooling to room temperature, 13.3 grams of Tinuvin 900 and 5 parts of toluene is added. For flow coating, further dilution with 30 parts of a 3:1 parts by weight mixture of Dowanol PM and diacetone alcohol is required. Polycarbonate samples are dip-coated with an acrylic primer containing an ultraviolet light absorbing agent which is cured for 20 minutes at 120° C. The primed polycarbonate samples are then dip-coated in the silane composition of this example. For flow coating, the sol can be further diluted with 80 parts of a 2:1:1 parts by weight mixture of 2-propanol, diacetone alcohol, and ethyl acetate. Spray application requires further dilution. This coating is cured for 90 minutes at 130° C. Results for coated and uncoated samples are compared in the following table.

                  TABLE II                                                         ______________________________________                                         Test     Duration  Measurement                                                                               Coated  Uncoated                                 ______________________________________                                         Bayer    300       % Haze     6       55-60                                    Abrasion cycles                                                                Taber    300       % Haze     3-4     >55                                      Abrasion cycles                                                                Taber    500       % Haze     6-9     >55                                      Abrasion cycles                                                                QUV-FS-40                                                                               Hours     craze/debond                                                                              2000    250 (10)                                                               (2)                                              QUV-B313 Hours     craze/debond                                                                              700-800 100 (10)                                                    (yellowness                                                                               (>3)                                                                index)                                                      ______________________________________                                    

The above examples are offered to illustrate the present invention. Various hydrolyzable alkoxides and ultraviolet absorbers such as hydroxybenzophenone and/or hydroxybenzotriazole compositions may be combined in a wide range of proportions and concentrations, so long as there is sufficient alkoxide to form an oxide network and sufficient UV absorber to provide desired,improvement in ultraviolet radiation resistance, as measured by the UV absorbence spectrum of the coating on a quartz substrate, without compromising the abrasion resistance of the coating. Various organic film-forming polymers may be included. These coatings are useful on a variety of substrates in accordance with the present invention, the scope of which is defined by the following claims. 

We claim:
 1. A silica-free optically transparent coating composition for protecting a substrate from ultraviolet radiation consisting essentially of:a. a partially hydrolyzed alkoxide of the general formula R_(x) M(OR')_(z-x) wherein R is an alkyl radical, M is selected from the group consisting of silicon, aluminum, titanium, zirconium, cerium and mixtures thereof, R' is a low molecular weight alkyl radical, z is the valence of M and x is less than z and may be zero; b. an organoalkoxysilane of the general formula R_(y) Si(OR')_(4-y) wherein R is an organofunctional radical, R' is a low molecular weight alkyl radical and y is at least one; c. an organic compound which absorbs ultraviolet radiation selected from the group consisting of hydroxybenzotriazole and hydroxybenzophenone; and d. polyvinylpyrrolidone.
 2. An optically transparent, ultraviolet radiation resistant and abrasion resistant coated article comprising a transparent rigid polymeric substrate and an ultraviolet radiation resistant and abrasion resistant coating which is the reaction product of:a. the hydrolytic polycondensation of a metal alkoxide of the general formula R_(x) M(OR')_(z-x) wherein R is an organic radical, M is selected from the group consisting of silicon, aluminum, titanium, zirconium, cerium and mixtures thereof, R' is a low molecular weight alkyl radical, z is the valence of M and x is less than z and may be zero; b. an organoalkoxysilane of the general formula R_(y) Si(OR')_(4-y) wherein R is an organofunctional radical, R' is a low molecular weight alkyl radical and y is at least one; c. an organic compound which absorbs ultraviolet radiation; and d. polyvinylpyrrolidone. 